1. Field of the Invention
The present invention relates to a valve timing control apparatus for controlling a valve timing of a valve, which is opened and closed by a camshaft according to a torque transmitted from a crankshaft in an internal combustion engine.
2. Description of Related Art
Conventionally, a known valve timing control apparatus includes a housing, which is rotatable with rotation of a crankshaft, and a vane rotor, which is rotatable with rotation of a camshaft. JP-A-2010-285918 (US 2010/0313835 A1) describes a valve timing control apparatus in which a rotation phase of a vane rotor is changed toward advance side or retard side relative to a housing by introducing working fluid into an advance chamber or a retard chamber which are separated from each other in a rotation direction by the vane rotor in the housing. The valve timing control apparatus has a control valve and a linear solenoid. The control valve controls the flow of working fluid relative to the advance chamber and the retard chamber based on a reciprocation of a spool in a sleeve. An output shaft of the linear solenoid drives the spool of the control valve.
JP-A-2005-45217 (US 2004/0257185) discloses such a linear solenoid having a pair of cylindrical stators that oppose to each other through an air gap in an axis direction and a needle that reciprocates integrally with the output shaft. The pair of cylindrical stators and the needle are accommodated in an internal chamber of a casing. An energized coil generates a magnetic flux that passes through a magnetic circuit, and the pair of stators forms the magnetic circuit together with the needle, so that the needle reciprocates on the inner circumference side. As a result, the control valve controls the flow of working fluid relative to the advance chamber and the retard chamber based on the reciprocation of the spool. Thus, the valve timing is controlled by the change of the rotation phase.
In a case where the linear solenoid is applied to the valve timing control apparatus, when the working fluid passes through the sleeve open to the linear solenoid, a part of the working fluid flows into the internal chamber of the casing from a bearing clearance of the output shaft, for example. In this case, if a foreign object such as metal powder having magnetic property (hereinafter referred as magnetic object) is contained in the working fluid, the magnetic object may stay in the air gap between the pair of stators, so that unnecessary short circuit may be generated in the magnetic circuit.
As shown in FIG. 14 of JP-A-2005-45217, the linear solenoid has a cylindrical coil bobbin that fits with an outer circumference side of the pair of stators in the internal chamber of the casing, so that the short circuit may be restricted from being generated. However, because the air gap is surrounded by the bobbin from the outer circumference side, the air gap has no outlet for the magnetic object. If the magnetic objects stays in the air gap, the function of restricting the short circuit is lowered. The short circuit causes a lowering in the responsivity of the spool that is driven by the output shaft, so that the responsivity of the valve timing control may be lowered.